The invention relates to a band-pass filter for electromagnetic radiation which consists of a membrane with slot structures arranged to provide for optimum filter selectivity.
In photometric tests or analysis in the infrared range, wide-band semiconductor detectors are utilized. In order to achieve the best possible spectral analysis and, consequently, determinations with regard to the objects to be analyzed, the large sensitivity ranges of the detectors must be divided into ranges with defined band width by means of band-pass filters.
In order to provide for optimal measuring range separation the band-pass filters should have a rectangularly shaped transmission profile. As a result of such a rectangular profile a sharply defined passing band would be obtained. With the flank between the suppressed spectral range and the passing band being infinitely large a band-pass filter would provide for ideal definitions. In addition to full definition (sharpness) an ideal band-pass filter provides for complete radiation passage in the pass-band and total absorption in the adjacent spectral ranges.
In practice, however, the definition of band-pass filters varies since the inclination of the flanks at the pass-band depends on the geometric dimensions of the filter. High definition, that is, a large inclination of the low-frequency flank, is particularly then necessary when the low-frequency end of the desired measurement range does not coincide with the sensitivity limit of the detectors.
In the publication "Interference filters for far infrared", Appl. Opt. 7 (1968), pages 1987 to 1996, R. Ulrich describes band-pass filters which consist of metal membranes with periodically arranged slot apertures. These band-pass filters are known as so-called resonance gratings.
In a publication "Kleinheubacher Berichte" No. 29 (1986) published by Fernmeldetechnisches Zentralamt, Postfach 5000, 6100 Darmstadt, Germany, pages 501 to 505, H. P. Gemund describes a process for manufacturing microstructures on a galvanic base layer disposed on a glass plate. The grating constant in this arrangement is above 34 .mu.m and the filter thickness is smaller than the grating constant. These band-pass filters have only relatively small thickness and, as a result, inadequate filtering characteristics.
From an article by S. T. Peng et al. "Theory of Periodic Dielectric Waveguides" published in IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-23, No. 1, January, 1975, pages 123 to 133, dielectric wave guides are known, which have periodically arranged slot structures. However the structures described therein are not suitable for band-pass filters.
It is the object of the present invention to provide a membrane-type band-pass filter with improved filter characteristics.